Most existing offshore oil and gas fields are drilled and produced from rigid structures which rest on the ocean bottom and extend upward to a work deck situated above the ocean surface. A key constraint in the design of such offshore structures concerns limiting the dynamic amplification of the structure's response to waves. Failure to minimize such dynamic amplification will diminish the fatigue life of the structure, and in extreme cases can result in the imposition of excessive loadings on key structural components. Avoidance of dynamic amplification is typically achieved by designing the structure to have rigidity sufficient to ensure that its natural vibrational periods are less than the shortest period of significant enery waves to which the structure will be exposed. For most offshore locations the shortest significant wave period is about seven seconds.
Hydrocarbon drilling and production structures designed in accordance with this approach have proved very satisfactory for most applications in water depths of up to about 300 meters. However, in water depths exceeding 300 meters, the quantity of structural steel required to maintain the fundamental natural vibrational period of a conventional rigid platform below the shortest significant wave period becomes an increasingly strong function of water depth. Because of this, most offshore hydrocarbon reservoirs in water depths much beyond 300 meters cannot be economically produced using a conventional rigid platform.
For deep water applications, it has been proposed to depart from conventional rigid platform design and develop platforms having a fundamental natural period greater than the range of periods of ocean waves containing significant energy. Such platforms, termed "compliant structures," do not rigidly resist waves and other environmental forces, but instead respond compliantly to these forces, undergoing significant lateral motion at the ocean surface either through sway (pivoting of the structure about its base) or bending (flexure of the structure along its length). The use of a compliant offshore structure effectively removes the upper bound on the sway or bending period, thus avoiding the most troublesome design constraint of rigid structures. This greatly reduces the increase in the volume of structural material, and hence cost, required for a given increase in water depth.
Because economic considerations have not yet warranted extensive exploitation of offshore hydrocarbon reserves in water depths greater than about 300 meters, the development of compliant structure technology is currently at a fairly early stage. However, several types of compliant structures have been designed and a few have been constructed and placed in service. One of the most promising concepts for achieving compliancy is incorporated in a proposed structure known as the compliant piled tower. The compliant piled tower is a slender, substantially rigid space-frame tower extending from the ocean floor to a position above the ocean surface. A drilling and production deck is supported atop the tower. Unlike a conventional platform, the tower is not rigidly tied to the ocean floor. This permits the structure to tilt about its base in compliant response to waves, wind, ocean currents and other lateral forces. The tower is stabilized against excessive sway by tubular steel piles which extend upward from positions surrounding the base to a pile attachment position located a preselected elevation above the ocean floor. In response to sway of the tower away from the vertical, the piles establish a righting moment acting at the point of pile attachment. This provides the stabilization necessary to restore the tower to a vertical orientation. One type of a compliant piled tower is detailed in an article at pages 20-25 of the March, 1986 edition of Ocean Industry magazine.
A key problem in the development of a practical compliant piled tower centers on the design of the stabilizing piles. As taught in the article cited above, the stabilizing piles are tubular steel elements driven into the ocean floor near the periphery of the tower base and extending upward to a significant elevation above the ocean floor, where they are rigidly secured to the tower. Elastic extension and compression of the tubular steel piles occurs in the course of the tower sway to establish the restoring force necessary to yield the requisite stability. A significant drawback of this arrangement is that it requires a large number of lengthy piles. This significantly increases the weight and cost of the structure. Moreover, in offshore locations combining harsh environmental conditions with relatively shallow water depths, it may not be practical to provide the compliant structure with stabilizing piles long enough to accommodate the necessary extension of the pile without exceeding the safe operating elastic limit of the steel or other material from which they are fabricated.
It would be desirable to develop a pile assembly for compliant piled towers and related offshore structures which provides the necessary compliancy and stabilization while being shorter and less expensive than the compliant pile assemblies proposed heretofore.